feat(tui): add per-locale i18n infrastructure and wire UI strings - #52000

Issue for this PR

Closes #37216

Tracking notes: #53857 (opened for this PR's issue-first link) was triaged by the repo bot as a duplicate of #37216 and is being closed as such; #51998 was auto-closed by the issue-compliance workflow before its body could be converted to the template structure. Both preserved for history.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adapts the TUI i18n groundwork from #48731 (by @neriousy, original commits preserved via merge) onto the current v2 branch, where it had fallen ~550 commits behind.

• i18n infrastructure in packages/tui: lazy per-locale dictionaries for 62 locales, language.t() / plural() / number() / duration() / date() helpers, useLanguage context, a language picker, and config.language support. No behavior change when no locale is configured (English default).
• Wiring: ~100+ components and feature plugins (dialogs, composer, session route, sidebar, diff-viewer, stats, toasts, notifications, …) now read user-facing strings through language.t(...) instead of hardcoded English.
• Conflict resolution against v2: kept v2's evolved APIs (theme tokens, surface(), dialog patterns, props.sources diff sources, fail/attempt helpers) and re-applied the PR's language.t(...) wiring on top; dropped PR-side features whose APIs v2 has since removed.
• Synced with current v2 again (merge commit, 216c311744): upstream meanwhile moved GUI features into built-in extensions; 10 tsx files conflicted (dialogs, session tabs, sidebar, subagents tab, …) and were resolved by keeping upstream's structural changes (new StringChoiceField, external connection flow, recently-closed-tabs menu, filter.active rename, removed server?/overlay? props and the unshare command) and re-wrapping their new user-facing strings: ~25 language.t(...) call sites re-applied, +6 keys (tui.details.submitAnswer/backToAnswers/editAnswerHint/selectAnswerHint, tui.dialogs.recentlyClosedTabs/noRecentlyClosedTabs — zh/zht translated, other locales English placeholder), −1 orphan key (tui.session.unshareSession, UI removed upstream).
• 7 new keys (tui.diff.turn*, tui.verbosity, tui.tabs.mode, tui.dialogs.pairHint, tui.dialogs.updating) added to en.ts and all 62 locale files; zh/zht carry real translations, other locales carry the English source string as placeholder — same convention as the rest of the PR.

How did you verify your code works?

• oxlint packages/tui/src packages/tui/test → 0 warnings, 0 errors (523 files)
• tsgo -b in packages/tui → exit 0
• After the v2 sync: bun test in packages/tui → 1504 pass / 12 skip / 11 fail; the failure set is baseline Windows-environment failures (symlink EPERM, atomic-rename file locks, footer rendering) including two new test files upstream added that hit the same symlink/rename limits — no logic regressions; tests touched by the conflict resolution pass in isolation
• After the v2 sync: tsgo -b in packages/tui → exit 0; prettier --check on all changed files → clean; test/i18n.test.ts → 64 pass (all locales cover the en key set with matching placeholders)
• bun run test:unit in packages/app → 930 pass / 0 fail (i18n parity tests included)
• locale key check (verifykeys.js) → ok: 63, bad: none
• repo-wide grep for conflict markers → none

Screenshots / recordings

Terminal UI change: default English output is visually unchanged; the one new visual element is the language picker dialog (unchanged from #48731). Can record a short demo on request.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR